1. Field of the Invention
The present invention relates generally to processes and packaging for high voltage integrated circuits (ICs), electronic devices and high voltage electronic circuits. More particularly, this invention relates to processes for fabrication and manufacturing of high voltage integrated electronics that are capable of voltage operation in a range of tens of volts to tens of thousands of volts.
2. Description of Related Art
High voltage integrated circuits (ICs), electronic devices and circuits find many applications. High voltage ICs, high voltage electronic devices and circuits may be used to replace bulky discrete electronic components such as individual high voltage transistors, resistors and transformers. Additionally, it is desirable to integrate these generally bulky discrete high voltage components directly on silicon, in a package, or on a printed circuit board (PCB) to achieve greater miniaturization, higher reliability and lower power consumption. Such high voltage ICs, electronic devices and circuits may also reduce the size and cost of current high voltage electronics that employ bulky electronics components by eliminating at least some “pick and place” operations required of a large number of electronic components. Furthermore, there is also the opportunity to use mass production facilities and processes that are commonly used for fabricating integrated circuits.
However, high voltage electronics require careful attention to physical layouts when formed on semiconductor substrates and when mounting on PCBs. This is because the high voltages may cause arcing to nearby components, or circuit traces and thereby cause malfunction and/or damage to the circuits. Breakdown voltage, Vbd, is a common measure of an electronic component's ability to tolerate high voltages. Electronic devices having a high breakdown voltage, Vbd, are more tolerant to device failure. Thus, it is desirable to have a high breakdown voltage, Vbd, in high voltage ICs, high voltage electronic devices and circuits.
Early approaches to integrating high voltage components were focused at the PCB level. For example, U.S. Pat. No. 5,699,231 to ElHatem et al. discloses isolating discrete electrical components on a PCB having slots or other cut out shapes to prevent charge migration on the surface of the board between high and low voltage nodes in combination with potting of the surface of the PCB. The slots or other cut out shapes allow the potting material to flow around and through the board and the electronic devices. However, the ElHatem et al. approach only achieves greater densities at the PCB level.
Other approaches have been taken to manufacture high voltage ICs. For example, U.S. Pat. No. 5,382,826 to Mojaradi et al. discloses the integration of a high voltage transistor by stacking any number of lower voltage transistors. As disclosed in the Mojarradi et al. patent, spiral and star shaped field plates may be used to space out the equipotential field lines to avoid voltage concentrations. Mojarradi et al. further discloses that the voltage range for the circuit may be increased by stacking several devices in series configuration. However, such series stacking configurations generally require high voltage resistors for correctly biasing gate voltages through a voltage divider network and other means. Even in integrated form, high voltage resistors are bulky. Thus, it would be particularly advantageous to have a process that also readily provides high voltage resistors for use in biasing high voltage transistors and other applications.
U.S. Pat. No. 4,908,328 to Hu et al. discloses a process for forming an oxide isolated semiconductor wafer which can include the formation of an associated high voltage transistor. The Hu et al. process includes bonding a first wafer to a second wafer using oxide, forming a groove through the oxide and back-filling with an epitaxially regrown semiconductor for placement of the high voltage devices. However, Hu et al. does not appear to disclose the formation of individual circuits on a substrate, separating the individual circuits on the substrate, dicing a module from the separated individual circuits, isolating the module, making connections on the module and back-filling to further isolate the individual circuits and to achieve high breakdown voltage, Vbd.
Another approach to packaging high voltage transistors is disclosed in U.S. Pat. No. 5,577,617 to Mojarradi et al. More particularly, the '617 patent discloses the use of a lead frame for mounting multiple individual transistors while isolating substrates to achieve electrically stackable high voltage transistors in a single package. The '617 patent also discloses that further isolation may be achieved by using an isolating epoxy to encapsulate the die mounted on the die mount tabs. The encapsulation provides further insulation because the dielectric constant of the encapsulant is greater than that of air.
U.S. Pat. No. 5,739,582 to ElHatem et al. discloses a method of packaging multiple high voltage devices in a multi-chip module. More particularly, the '582 patent discloses the mounting of any number of high voltage chips in a cavity of a package, electrically connecting the high voltage devices together and to a lead frame. The '582 patent further discloses the use of three coatings to provide further isolation. First, a non-conductive epoxy for mounting the devices to the cavity floor is applied. Second, a thin layer of polyimide or Dupont™ Pyralux™ is applied to the downward side of each of the devices. Finally, a die coating of Q1-6646 Hipec™ Gel die coat material is applied to the upward side of each chip to suppress arcing. However, the '582 patent warns that “it is essential to use all three coating materials, the polyimide 68, the die coating 70, and the non-conductive epoxy 72 to achieve and sustain the high operating voltages. If even one of the coating materials is missing or defective the voltage sustainable by the packaging drops dramatically and reliability of the parts will be severely downgraded.” Col. 4, II. 6-11.
Accordingly, there still exists a need in the art for processes and packaging for integrating lower voltage electronic devices to form high voltage integrated circuits, high voltage electronic devices and circuits.